The Crystal and Mineral Vault

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Boulder Opal: Why the Ironstone Matrix Is Not a Flaw but the Whole Point

by Laura Konst

Boulder Opal contains no colour. Its extraordinary play-of-colour, shifting through blues, greens, reds, and purples as the viewing angle changes, is produced entirely by the diffraction of white light through a regular array of nanoscale silica spheres within the stone. No pigment, no impurity, no chemical colour mechanism: just physics at the nanoscale. This guide explores the diffraction science, the Queensland ironstone geology, the distinction between natural Boulder Opal and assembled doublets and triplets, and why the dark ironstone matrix is not a compromise but the feature that makes the colour play so vivid.

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